Angiotensin converting enzyme (ACE) converts angiotensin I (AI) to angiotensin II (A II) and inactivates bradykinin (BK) by its dipeptidyl-carboxypeptidase activity. It is thus possible that ACE has an important role in the regulation of blood pressure in normal physiology. The factors modulating serum ACE activity and its relationship to tissue ACE activity are unclear. No abnormalities of serum ACE activity have been reported in hypertensive subjects. Serum ACE-activity has previously been reported to be supranormal in sarcoidosis and Gaucher's disease but the increases in serum ACE-activity were not associated with elevations of blood pressure. Serum ACE-activity was measured in normal volunteers, hypertensive subjects on 9 mEq and 109 mEq sodium diets and in patients with sarcoidosis. The levels of serum ACE activity were similar in normal and hypertensive subjects and were not influenced by dietary sodium. Serum ACE-activity in patients with sarcoidosis varied with the activity of the disease.